Study of the Indian Lithosphere using Geopotential Data

Mita Rajaram, V.C.Erram, S.P.Anand, B.N.Shinde and Nisha Nair

Satellite, aero ground and marine magnetic and gravity data are utilized to study the anomalies in the Indian lithosphere and to relate these to geotectonic processes. Gravity and Magnetic signatures of the lithosphere have imprinted on them the tectonic history of the plate. These signatures depend on the density and susceptibility of the rocks and the magnetization acquired during the past. By coupling ground, aeromagnetic and satellite crustal anomalies is possible identify the source fields in the lithosphere. The way we see these signatures depend upon the altitude / platform of observation and the data spacing. Thus a combination of ground, aero, marine and satellite magnetic data together with available gravity data could enable us to map the lithospheric thickness of the Indian sub-continent and thereby help us look back in time and reconstruct the plate history. Understanding the configurations of past super continents is of paramount importance. The formation and dispersal of super continents has had a marked effect on past changes in ocean circulation patterns and hence on Earth's climate. Major mineral provinces on one continent may have as yet undiscovered corollaries on another, once adjacent continent and hence plate reconstruction is important in economic point of view as well.

Current Research

Seismotectonics of the Kutch region from aeromagnetic, gravity, GPS and seismological data.

Integrated Geophysical Studies bearing upon the Crustal and Lithospheric evolution of the Bastar Craton with special emphasis on kimberliteemplacement.

Modeling of the satellite derived Free Air gravity and ship borne magnetic data to study the evolution, structure and configuration of Western Continental Margin of India with special Emphasis on Laxmi Ridge and Laxmi Basin.

Ongoing/Completed External Projects

“Satellite Geoid/Gravity Modelling for Lithospheric Studies” under Meteorology Oceanography Program (MOP) funded jointly by Indian Space Research Organization (ISRO) and Department of Ocean Development (DOD). 2004-2006.

“Studies on Marine Lithosphere” under MOPII (Costal and Geological Oceanography) funded jointly by Indian Space Research Organization (ISRO) and Department of Ocean Development (DOD), 2009-2014.

Instruments

GSM 19T Proton Precession Magnetometer

GSM 19 Over Hauser Magnetometer

Magnetic Gradiometers

VLF Unit

Handheld GPS

Major findings

For the first time a Composite Magnetic Anomaly Map of India and its Adjoining regions (Figure.1) were brought out by IIG integrating all the available ground, airborne and marine magnetic data (Rajaram et al., JGSI,68,569-576,2006)

From the aeromagnetic image map of India (up to 25° N), we find that the main magnetic sources within the cratons (Dharwar, Bastar and Singhbhum) are related to iron ore belts, schist belts and dyke systems while the sources within the mobile belts (EGMB, SGT and the CITZ) are due to the exhumed crust reflecting the high-grade granulite belts like charnockites. Contrary to the belief that the Central Indian shear defines the edge of the Central Indian craton, we find that the Sukinda thrust merges with the Tan shear to demarcate the edge of the Bastar and Eastern Ghat blocks by a shear that extends for approximately 1000 km in length, coined Main Peninsular Shear. The magnetic data helps to unravel the subsurface, below the sediments and trap cover. The proxy heat flow map generated from the calculated cuire depths matched reasonably well with the observations. The heat flow values in the cratons were low and they were high within the EGMB and CITZ (Rajaram and Anand, EPS, 55, e1–e4,2003; Rajaram et al., JGSI,68,569-576,2006; Anand and Rajaram, IAGR Memoir 10, 233-242, 2007).

The aeromagnetic data analysis over the part of the Singhbhum uranium province (Figure.2) identified a shear zone whose magnetic sources were lying at a depth of 200m (from Euler solutions) which depicted a similar magnetic signature and susceptibility as those of the Singhbhum Shear Zone where uranium is being mined. It appeared to be the subsurface contact between high-grade metamorphic rocks (amphibolite facies) to the north and the greenschist facies low-grade metamorphic rocks to the south. This newly identified shear zone, also characterized by radiometric anomalies, possibly indicates a zone of concealed uranium deposit that can be explored in the future (Anand and Rajaram, EPS,58, 1099–1103, 2006)

Aeromagnetic and magnetotelluric investigations were carried out in the Parnaiba basin in Piaui State of North-East Brazil for assessment of groundwater resources. Magnetic data revealed several subsurface small-scale faults and intrusives that are conducive to be potential groundwater resource regions. Two-dimensional inversion of MT data along two profiles shows that the sedimentary basin is having an average thickness of 2-3km and is conductive (100–150 Ω m). The mapped sedimentary basin largely manifests the zone of potential sedimentary aquifer having moderate resistivity of 50–250 Ω m located at relatively shallow depths. The identified aquifer zone is believed to have links with the Parnaiba River flowing at a distance of about 300 km NW of the basin (Chandrashekar et al,. Jour,Appl. Geophy.68, 269–281,2009)

From the analysis of magnetic data, it was confirmed that high-grade charnockitic rocks on surface and sub-surface flank the shoulders of the Godavari Graben on eithcr side. Sileru Shear Zone (SSZ) is identified as the contact of the Bastar craton and the Eastern Ghat Mobile Belt (EGMB). The Eastern Ghat is divided into two blocks: Block-N north of Srikakulam is devoid of magnetic sources while the charnockitic rocks are the main magnetic carriers in Block-S. The difference In magnetic characteristics of the two blocks has been attributed to the difference in metamorphic history. The northern block has an over print of amphibolite facies metamorphism while southern block depicts granulite facies metamorphism, It was also found that the exhumation process in the EGMB has a differential rate (Anand and Rajaram, Gond.Res., 6, 859-865, 2003)

In the Narmada-Son lineament, Gondwana rocks are present underneath the traps between Baruch and Bharwaha and are controlled by the Narmada River Fault (F1) to the north. The magnetic signatures to the north and south of F1 are different, implying that the evolutionary history of the continental crust north and south of fault F1 is different and the trap flows on either side are not coeval or they have different thicknesses. (3) In the region between Hoshangabad and Narasimapura, the crust is made up of two units: an EW shallow unit superposed on a deeper NW-SE unit possibly associated with the extension of the Gondwana Godavari Graben towards the northwest (Anand and Rajaram, EPS, 56, e9–e12, 2004).

From ground magnetic studies conducted over the pull apart Krishna-Godavari basin and Mahanadi basin we found that both the basins are formed of two structural units: a shallow NE-SW unit and a deeper NW-SE unit. These two structural features are associated with two different tectonic events The shallow NE-SE trends represent the horst and graben structure related to the rifting and rifting of India from Gondwanaland. The deeper NW-SE trends are related to the Godavari and Mahanadi Gondwana Grabens formed as a result of the separation of Bastar crton from Dharwar and Singhbhum craton respectively. Both these deeper trends are found the extend offshore underneath the coastal basin (Rajaram et al, Gond.Res.3, 385-393,2000, Anand et al., JGSI, 60,283-291,2002).

Crustal model derived from the combined analysis of high resolution satellite derived free air gravity (FAG) and magnetic data 85° E ridge suggest that the ridge has a different nature to the north and south of the 15N latitude (Figure.3). Above 15N the ridge appears to be a geo-morphological feature within the sediments above the basement. Below 15N, the ridge appears to be oceanic; the reverse magnetization associated with the seafloor spreading anomalies lying on the 85°E Ridge belongs either to the Albianperiod or the anomalyA34/A33. Hence the 85°E Ridge would be younger than these seafloor spreading anomalies and would have formed due to horizontal compressional forces of the lithosphere preceding development of the subduction zone at the Andaman trench (Anand et al., Tectophy., 478, 100–110, 2009)

Depth to bottom of the magnetic sources, in the Indian sub-continent, was calculated from the MF5 model of the lithospheric field utilizing an iterative forward modelling approach and also from aeromagnetic data using spectral analysis. The derived Curie isotherm pattern (Figure.4) and trends is in broad conformity with the regional structural trends of the major tectonic units within the Indian subcontinent. The calculated Curie isotherm is shallow in the mobile belts and deeper in the cratons. A thick magnetic crust is consistent with stable continental regions while thin magnetic crust may conform to tectonically active regions, often associated with higher heat flow and earthquake epicentres (Rajaram et.al.,EPSL,. 281 147–158, 2009)

The Crustal Thickness Map derived from MF5 lithospheric model of CHAMP, shows that the continental crust in the Andaman-Sumatra region is associated with a thick magnetic crust. The FAG and its transformation identified a major thrust zone, coined ANDAMAN Thrust was depicted between the Andaman trench and West Andaman Fault. Signature of this thrust is evident in magnetotelluric as well as seismic data. All major deep focus earthquakes lie within this thrust and WAF. A zone of deep Euler solutions is identified, south of the Andaman Thrust (AT). A deep seated NE-SW feature was identified south of AT that appears to reduce the probability of occurrence of high magnitude (>6.3) earthquakes towards the north.